The character is a cult legend amongst anime and manga fans, so why aren't his films more widely available? There's only Castle Of Cagliostro and The Secret Of Mamo available in the UK on DVD, but other than that, you'll be struggling to find anything else...
Not a big fan of anime in general, but since a close friend of mine introduced me to Lupin III, I've been obsessed with it.
And I've already purchased close to 20 DVDS, with many more to go.
Ideally, we would live in a perfect world where all of the Lupin III DVD's were easily obtainable, and all of them were dubbed by Tony Oliver, Michelle Ruff, Richard Epcar, Lex Lang, and Jake Martin, who dubbed the "Red Jacket" series for Adult Swim, because (IMHO) they nail the characters more than any other English speaking actors.
But, alas, we don't live in a perfect world, so we have to grad the DVD's, from whichever company is licensing them and whichever actors are dubbing them, when we can.
The Lupin III franchise is even more convoluted (in terms of rights/licensing) than the Batman TV series from the 1960s... at least our favourite gentleman thief has had his escapades released on DVD. OK, you might need a multi-region/region-free player, but it's a small price to pay.